When air flows over a golf club head, viscous forces near the surface of the club head create a velocity gradient from the surface to the free stream region. Accordingly, air flow velocity near the surface may be relatively slow and gradually increases toward the free stream velocity, which is the air flow region where air velocity is not influenced by the club head. This velocity gradient region is called a boundary layer. Flow separation occurs when the boundary layer travels on the golf club head far enough against an adverse pressure gradient that the air flow velocity in the boundary layer relative to the surface of the club head falls almost to zero. The air flow becomes detached from the surface of the club head and takes the form of eddies and vortices. Flow separation may result in increased drag, which may be caused by the pressure differential between the front and rear surfaces of the club head. The increased drag may reduce the speed of the club head, which in turn may lower the velocity of a golf ball that is struck by the club head.